1. Field of the Invention
The present invention relates to a process for densifying ceramic parts, and particularly to a process for densifying sintered ferrite ceramic parts.
2. Description of the Prior Art
The term ferrite generally denotes a class of non-metallic, ceramic ferromagnetic materials (MFe.sub.2 O.sub.4) having a spinel-crystal structure. The M in the formula popularly is MnZn or NiZn. Ferrites have desirable magnetic properties such as a high resistivity relative to ferromagnetic metals, low hysteresis losses and good permeability making them particular useful in magnetic heads for video, audio, data storage and other related applications.
Generally, ferrites are manufactured by mixing measured amounts of metal oxides or metal carbonates and then pulverizing the mixture. The pulverized mixture is calcined and then wet-milled. The resulting wet-slurry is then spray-dried with a binder to produce a ready to press powder Ferrite parts are formed by pressing, molding or extruding.
After forming, the parts are sintered at temperatures of about 1,000.degree. C. before the sintered parts are further densified to approximately theoretical density in a hot isostatic pressing (hipping) procedure. Prior art densifying processes "hip" the sintered ferrite part in a sealed totally-inert atmosphere, typically of argon. See, for example, U.S. Pat. No. 4,440,713, issued Apr. 3, 1984, entitled "Process for Making Fine Magnetic Ferrite Powder and Dense Ferrite Blocks," for a discussion of hipping nickel-zinc ferrite in a pure argon atmosphere.
Generally, hipping a ferrite or other ceramic part includes the steps of disposing a sintered ferrite part in a pressure vessel at ambient temperature and at ambient pressure, creating a sealed atmosphere within the vessel, heating the part to a soaking temperature while pressurizing the part to a soaking pressure, soaking the part at the soaking temperature and pressure, and then cooling the part to ambient temperature and pressure.
A major drawback encountered with prior art hipping procedures is that, during hot isostatic pressing, the ferrite undergoes chemical changes which adversely affect the magnetic properties of the resulting hipped ferrite part. The inventors believe that such changes are caused substantially by hipping in the totally inert atmosphere. In a pure argon atmosphere at a high temperature (e.g. 1185.degree. C.) and a high pressure (e.g. 15 KPSI), the inventors believe that the Fe ions of the ferrite are reduced, and that the reduced ions then change their sites in the crystal lattice of the ferrite. Thus, the chemistry of the hipped ferrite part is changed from that before hipping. For a discussion of various mechanisms which may be involved in this chemical change, see Van Vlack, Elements of Materials Engineering, Third Edition,--(Addison-Wesley Publishing Company, 1975).
An additional drawback of the prior art hot isostatic pressing procedure is the necessity for closely surrounding the sintered ferrite or other sintered ceramic with a protective matrix (or "frit"). The frit consists of sintered ceramic pieces having the same chemical composition as the sintered ceramic material to be hipped. The frit is utilized in a prior art attempt to minimize the previously discussed chemical changes.